Electric power distribution networks are used by the electric utilities to deliver electricity from generating plants to customers. Although the actual distribution voltages will vary from utility to utility, in a typical network, three-phase power at high voltage 345,000 volts phase-to-phase (345 KV) is delivered to multiple high voltage substations at which trans-formers step this high voltage down to a lower three-phase voltage 115 KV. Multiple transmission substations further lower this voltage to 69 KV. This 69 KV three-phase power then feeds multiple distribution substations whose transformers further step down this voltage to distribution voltage (12,470 volts phase-to-phase) and separate power into three single-phase feeder cables. Typically, these feeder cables operate at 7,200 volts phase-to-ground. Each of these feeder cables branch into multiple circuits to power a plurality of local pole-mounted or pad-mounted transformers which step the voltage down to a final voltage of 120 and 240 volts for delivery to commercial and residential customers.
The instantaneous phases of the three conductors in a three-phase system are separated by 120 degrees. A phase attribute of A, B, or C is typically assigned to each of the three conductors to identify them. The initial assignment of phase attribute to each of the three conductors typically takes place at a transmission or distribution substation and this assignment is somewhat arbitrary. The attributes assigned at the substations become known as the tagging reference phases for that substation because the goal is to consistently tag, mark, or identify each conductor with its proper phase attribute throughout the substation's distribution region.
Utilities have many reasons for accurately identifying the phase of each conductor in their utility. Examples are load balancing to reduce neutral current flow, faster service restoration after outages, and for distribution automation purposes.
Most currently available phase identification instruments use GPS timing signals to obtain instantaneous phase measurements at a reference location and field location at the same instant of time. The phase attribute at the reference location is known which allows the phase attribute at the field location to be determined. For example, if the reference location phase attribute is B and the instantaneous phase measured at both locations are the same, then the field location phase attribute is also B. If the instantaneous field location phase is leading or lagging the instantaneous reference location phase by 120 degrees, then the field location phase attribute is either A or C depending on the utilities known phase rotation.
To identify the field location phase attribute, the instantaneous phases at both locations have to be compared. Current phase identification instruments differ primarily in the method they each use to communicate the instantaneous phase reading between the reference location unit and the field location unit. Most manufacturers implement a real-time communication system using cell phones. Piesinger's, U.S. Pat. Nos. 6,667,610 and 7,031,859 describe a phase identification method that does not require real-time cell phone communication.
To phase identify high voltage transmission and distribution conductors, the phase identification instrument is typically attached to an insulated fiberglass stick. Short sticks are called hot sticks and long telescoping ones are called extendo sticks. The purpose of these sticks is to place the phase identification instrument in close proximity to the conductor being phase identified and to protect the lineman from the high voltage. To reach high overhead transmission and distribution lines, long extendo sticks on the order of 30 to 50 feet may be required.
Maneuvering the phase identification instrument, on the end of a long extendo stick, into position is difficult and time consuming. It is especially difficult in a strong wind because long extendo sticks are not very rigid. Gusting winds can cause the phase identification instrument to sway wildly making it nearly impossible to hold it into position next to a high overhead cable. However, even using short hot sticks is inconvenient because it takes time to attach the phase identification instrument to the stick and hot sticks have to be carefully stored and maintained to remain effective as an insulator.
Accordingly, it is the object of the present invention to provide a new and improved method and apparatus for phase identifying high voltage conductors without the need for hot sticks or extendo sticks.